CSU 2018 Atlantic Hurricane Season Forecast: Slightly Above-Average

Above: The strongest Atlantic hurricane of 2017, Hurricane Irma, as seen by the MODIS instrument on September 5, 2017. At the time, Irma was at peak strength, a Category 5 storm with 180 mph winds. Irma was downgraded slightly in the post-season report by the National Hurricane Center, which judged that the peak sustained 1-minute winds of the storm were 180 mph, not 185 mph, as originally thought. Image credit: NASA.

A slightly above-average Atlantic hurricane season is likely in 2018, said the hurricane forecasting team from Colorado State University (CSU) in their latest seasonal forecast issued April 5. Led by Dr. Phil Klotzbach, with coauthor Dr. Michael Bell, the CSU team is calling for an Atlantic hurricane season with 14 named storms, 7 hurricanes, 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 130. The long-term averages for the period 1981 - 2010 were 12 named storms, 6.5 hurricanes, 2 intense hurricanes, and an ACE of 92. The CSU outlook also calls for a 63% chance of a major hurricane hitting the U.S. in 2018 (long term average is 52%), with a 39% chance for the East Coast and Florida Peninsula (long term average is 31%), and a 38% chance for the Gulf Coast (long term average is 30%). The Caribbean is forecast to have a 52% chance of seeing at least one major hurricane (long term average is 42%).

Five years with similar pre-season February and March atmospheric and oceanic conditions were selected as “analog” years that the 2018 hurricane season may resemble. These years were characterized by weak La Niña to weak El Niño conditions during August-October, but with a wide variety of tropical and North Atlantic sea surface temperature (SST) patterns, due to the large uncertainty as to what the Atlantic SSTs will look like this summer and fall:

Mixed signals on El Niño/La Niña phase and SSTs

The CSU team cited two main reasons why this may be a slightly above-average hurricane season:

1) The current weak La Niña event in the Eastern Pacific appears likely to transition to neutral conditions over the next several months, but it is not anticipated that there will be a significant El Niño event this summer/fall. If El Niño conditions are present this fall, this would tend to favor a slower-than-usual Atlantic hurricane season due to an increase in the upper-level winds over the tropical Atlantic that can tear storms apart (higher vertical wind shear).

Sea surface temperatures (SSTs) were 0.7°C below average during March in the so-called Niño 3.4 region (5°S - 5°N, 120°W - 170°W), where SSTs must be at least 0.5°C below average for five consecutive months (each month being a 3-month average) for a weak La Niña event to be declared (and atmospheric conditions must also be consistent with La Niña). However, these temperatures have warmed 0.1°C since January, and in their latest March 8 monthly advisory, NOAA's Climate Prediction Center (CPC) predicted that the current weak La Niña event that began in August 2017 was near its end, with a 55% chance that it will transition to a neutral state by May. The Australian Bureau of Meteorology declared La Niña officially over in the March 13 installment of its biweekly report. The bureau uses a more stringent threshold than NOAA for defining La Niña: sea-surface temperatures in the Niño3.4 region of the tropical Pacific must be at least 0.8°C below average, vs. the NOAA benchmark of 0.5°C below average.

There is considerable uncertainty with the future state of El Niño during the 2018 hurricane season. The latest predictions from a large number of statistical and dynamical El Niño models show a large spread by the peak of the Atlantic hurricane season in August-October. About 1/3 of all forecast models are calling for El Niño conditions, with all but one of the remaining models calling for neutral conditions.

2) The western tropical Atlantic is anomalously warm right now, while portions of the eastern tropical Atlantic and far North Atlantic are anomalously cool. Consequently, the Atlantic Multi-decadal Oscillation (AMO) index is near its long-term average. A positive AMO is typically linked with above-average Atlantic hurricane activity; a negative AMO is typically associated with below-average activity.

As always, the CSU team included this standard disclaimer:

"Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them. They should prepare the same for every season, regardless of how much activity is predicted."

Figure 1. Departure of sea surface temperature (SST) from average for late March 2018. SSTs in the hurricane Main Development Region (MDR) between Africa and Central America were below average in the eastern Atlantic, and above average in the Caribbean. Virtually all African tropical waves originate in the MDR, and these tropical waves account for 85% of all Atlantic major hurricanes and 60% of all named storms. When SSTs in the MDR are much above average during hurricane season, a very active season typically results (if there is no El Niño event present). Conversely, when MDR SSTs are cooler than average, a below-average Atlantic hurricane season is more likely. The SST pattern above is a mixture of signals of both the positive and negative phases of the Atlantic Multidecadal Oscillation (AMO) pattern. The AMO has generally been in a positive phase since 1995, which has been associated with an active hurricane period. Image credit: NOAA/ESRL.

How good are the April forecasts?

April forecasts of hurricane season activity are low-skill, since they must deal with the so-called "spring predictability barrier." April is the time of year when the El Niño/La Niña phenomenon commonly undergoes a rapid change from one state to another, making it difficult to predict whether we will have El Niño, La Niña, or neutral conditions in place for the coming hurricane season. Last year’s CSU April forecast called for a slightly below-average Atlantic hurricane season for 2017, with 11 named storms, 4 hurricanes, 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 75. This forecast ended up being far too low, as the season actually had 17 named storms, 10 hurricanes, 6 major hurricanes, and an ACE of 226. The next CSU forecast, due on May 31, is the one worth paying attention to. Their late May/early June forecasts have shown considerable skill over the years. NOAA issues its first seasonal hurricane forecast for 2018 in late May, with an update in August.

Figure 2. Comparison of the percent improvement in mean square error over climatology for seasonal hurricane forecasts for the Atlantic from NOAA, CSU and Tropical Storm Risk (TSR) from 2003-2017, using the Mean Square Skill Score (MSSS). Values less than zero indicate that pure climatology does a better job than the forecast. The figure shows the results using two different climatologies: a fixed 50-year (1951 - 2000) climatology, and a 10-year 2008 - 2017 climatology. Skill is poor for forecasts issued in December and April, modest for June forecasts, and moderate-to-good for August forecasts. Using this methodology, TSR has had the best seasonal forecasts. Image credit: Tropical Storm Risk, Inc. (TSR).

TSR predicts a slightly below-average Atlantic hurricane season

The April 5 forecast for the 2018 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for a slightly below-average Atlantic hurricane season--about 15% below the long-term (1950-2017) norm and the recent 2008-2017 ten-year norm. TSR is predicting 12 named storms, 6 hurricanes, 2 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 84 for the period May through December. The long-term averages for the past 68 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 103. TSR rates their skill level as low for these April forecasts--just 2 - 7% higher than a "no-skill" forecast made using climatology. TSR predicts a 32% chance that U.S. landfalling ACE index will be above average, a 25% chance it will be near average, and a 43% chance it will be below average. They project that two named storms and one hurricane will hit the U.S. The averages from the 1950-2017 climatology are three named storms and one hurricane. They rate their skill at making these April forecasts for U.S. landfalls at 0% - 4% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects one tropical storm and no hurricanes. Climatology is one tropical storm and less than 0.5 hurricanes.

TSR’s main predictor for their statistical model of Atlantic hurricane activity is the forecast July - September trade wind speed over the Caribbean and tropical North Atlantic. Their model is calling for trade winds 0.49 m/s faster than average, due to the anticipated neutral El Niño/La Niña conditions during the summer/autumn of 2018. Stronger than normal trade winds during July-August-September are associated with less spin and increased vertical wind shear over the hurricane main development region, factors that reduce hurricane frequency and intensity. The next TSR forecast will be issued on May 30.

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